Marine seismic exploration of earth formations located below a body of water, usually offshore, is a well known technique of prospecting for hydrocarbons or other natural resources. Marine seismic exploration also can be used to obtain engineering survey data respecting shallow formations for siting and foundation design of offshore structures such as jack up rigs or permanent platforms for drilling and production operations.
In principal and in theory, seismic prospecting is relatively simple. A pulse of acoustic energy is produced which travels downward into the earth and is reflected back upward at interfaces between strata which have acoustic impedance mis-matches. The arrivals of reflected acoustic waves are detected by hydrophones and recorded by suitable instrumentation. By suitable calculations (including source signal deconvolution) or other processing of the recorded data, the depth, arrangement and thickness of the various underground formations as well as other significant characteristics may be determined with reasonable precision, and such information used to predict where hydrocarbons might be found by a exploratory well.
In actual practice however, the detection and data processing of the reflected acoustic energy data in order to produce reliable predictions is extremely complex and quite difficult. Each seismic source produces an energy signal having unique characteristics, commonly called its signature. In deconvolution the signature characteristics are used to adjust the recorded data for those known imperfections in the seismic signal. Separating a true reflected seismic signal from noise or other signal echoes present in the recorded data is an extremely difficult task and requires a great deal of skill and expertise. Furthermore, the characteristics of the acoustic pulses or signals that are actually generated by the seismic source can greatly increase the difficulty of sensing or detecting the proper reflected energy. False detection of the reflected energy will, or course, render the seismic determinations based on that information incorrect.
Numerous efforts and approaches have been made to improve the characteristics or signatures of seismic pulses or source signals in order to enhance the seismic exploration process. For example, Prescott U.S. Pat. No. 1,998,412 discloses the idea of distributing the explosive elements of a land seismic source vertically in a borehole and exploding them in succession from top to bottom. The objective was to cancel or reduce the amplitude of so-called "trailer" waves and improve the signal-to-noise ratio. Another U.S. Pat. No. 4,721,180 (Haughland et al) discloses a marine seismic source having vertically spaced subarrays of air guns which are fired shallowest first and deepest second. The arrangement is intended to attenuate both the bubble pulse oscillations and the "ghost" pulse arrivals which occur even in arrays that are "tuned" by selection of air gun sizes and spacing. Another related disclosure is a publication entitled "Three-Dimensional Air Gun Arrays", by Smith, G. C. published in Marine II, 1984. This publication focuses on increasing the efficiency of arrays of air guns (source strength per amount of air used) towed at different depths and fired at different times. The delay times are equal to depth difference divided by the velocity of sound through water. The air gun array proposed by Smith has a total volume of 5,560 cubic inches and employs four identical subarrays of 7 different sized guns. Each subarray was 19 meters long with larger guns near the front and smaller guns at the back. The subarrays were positioned on both the port and starboard sides of the towing vessel to form a wide array. Both uniform and nonuniform (coalesced) spacing of the different chamber size air guns on each side was employed. The four subarrays were towed at depths ranging from 5.4 to 11.0 meters with a sequenced firing time range of 0 to 3.75 ms. The timing of gun firing was sequenced to achieve in phase summation of the primary output signal. Other publications having overall relation to the problems involved in marine seismic exploration are reviewed in our U.S. Pat. No. 4,956,822 which is incorporated hereby by references.
Our U.S. Pat. No. 4,956,822 discloses a compact tricluster source array configuration or geometric arrangement for providing a tapered, heavy center, point source seismic signal useful in engineering type surveys when the guns are simultaneously discharged at a relatively shallow depth (1 to 3 meters). The disclosed geometric arrangement of the tricluster array employs eight air guns of equal volume size in a paired or clustered arrangement. The identifying number used after "tricluster" denotes the total firing chamber volume of the eight air guns employed. For example, the tricluster 80 which has a total of eight 10 cubic inch guns, provides a total chamber volume of 80 cubic inches, and the tricluster 160 uses eight 20 cubic inch guns for a total diameter volume of 160 cubin inches. The tricluster 320 employs eight 40 cubic inch guns for a total volume of 320 cubin inches. As each of the eight (8) air guns forming the tricluster are preferably substantially identical in construction, and have the same firing chamber volume size and are operated at the same air pressure, the output characteristics of each gun is substantially identical to all of the other air gun sources employed in the tricluster array.
The geometric arrangement of the tricluster array may be described generally as a symmetrical 2.times.4.times.2 configuration. A front pair of parallel guns are followed by a center cluster of 4 parallel guns and a rear pair of parallel guns. The center cluster of 4 guns is constructed by upper and lower pairs disposed midway between the front and rear pairs. In the '822 patent the horizontal spacing between paired guns is 20 in. and the distance between the center point of the front and back pairs and the center point of four gun cluster is about 5 feet or 60 in. The front and rear pairs of guns are disposed in a reference horizontal plane, and the upper and lower pairs of air guns in the center cluster are vertically spaced by 20 in. Thus the upper horizontal plane of the upper pair is 10 inches above the reference horizontal plane, and the horizontal plane of the lower cluster pair is located 10 in. below such reference plane. The compact, close spacing of the guns insures their operating interaction.
In forming the measured or calculated output signal (far field signature) of the tricluster or other array, the individual output signals of the air guns are combined. Whether an in-phase combining with magnitude reinforcement occurs or an out-of-phase combining with destructive interference of the magnitude occurs, signature is an extremely complex determination and is dependent on a number of factors including the length of the combining path, the wave length and the speed or velocity of the output wave or pulse. The measured tricluster 80 array far field signature or output signal is illustrated in FIGS. 6 and 9 of applicants' U.S. Pat. No. 4,956,822 while that of the smaller total chamber volume tricluster 32 is illustrated in FIGS. 7 and 8. In producing all of the measured far field signature signals all eight guns were fired simultaneously. For the tricluster 80 the amplitude characteristic was 7.1 bar-meters and a primary-to-bubble ratio of 94.9 was obtained, while that of the tricluster 32 was 4.0 bar-meters and a primary-to-bubble ratio of 60.0. The slight differences in measured data for each tricluster size is due to the speed of measurement sampling and to the bandwidth filter range employed (FIG. 6 is 27 to 256 Hz while that of FIG. 9 is 115 Hz to 1,024 Hz). Although the array shown in the '822 patent works quite well in practice, applicants have discovered a technique for firing the individual pairs of air guns in a manner that maximizes the acoustic energy in a useable range of seismic frequencies.
In the prior art as exemplified by the Smith publication and the Haughland patent noted above, sequential firing of vertically spaced subarrays is suggested as a way to suppress "ghost" reflections and improve the peak-to-bubble ratio. Specifically, the shallowest guns are fired first, the next shallowest second, and so forth, with a time delay equal to depth difference divided by the velocity of sound in water. In effect, the time delays are used to hopefully match the transit velocities in the direction in which transmission is desired and to attenuate the seismic signal in other directions. Applicants have discovered that the foregoing approach does not maximize the sonic energy produced by an array in a frequency range that is the most useful in seismic exploration, and that an overdelay is more effective in shaping a seismic pulse which has minimum attenuation in the subsurface formations. According to this invention, a delay time is used which insures that the firing of guns at any level occurs after the initial wave front due to forcing of the upper guns has passed that location.
One object of the present invention is to provide an array of seismic source air guns that are fired in a manner to shape the output energy wave so the preponderance of the usable energy produced is concentrated in a useful range of frequencies.
Another object of the present invention is to fire an array of air guns in a manner which controls the output wave energy such that it is predominantly in a usable frequency spectrum.
Yet another object of the present invention is to provide a firing sequence for a plurality of air guns which reduces the frequency range of the usable seismic bandwidth.